[#52201615] Make env template backwards compat
[#52201615] Make env template backwards compat

Referencing `scope` has a hash doesn't work in Puppet 2.7, which Vagrant is
still running for `puppet apply`:

Failed to parse template govuk/etc/environment.erb:
  Filepath: /var/lib/gems/1.9.1/gems/puppet-2.7.19/lib/puppet/parser/scope.rb
  Line: 459
  Detail: undefined method `[]' for Scope(Class[Govuk::Envsys]):Puppet::Parser::Scope
 at /tmp/vagrant-puppet/modules-0/govuk/manifests/envsys.pp:31 on node backend-1.backend

Revert to the old method of `scope.lookupvar()`. Maintain backwards
compatibility for unbound variables, which will return `nil` on 3.x and
`:undefined` on 2.7

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet